             (j/period 0 :months)
             (j/period 0 :days)))
 
-     (is (j/period? (j/period)))))
+     (is (j/period? (j/period)))
+     (is (not (j/period? nil)))))
 
 (deftest operations
   (testing "duration"
+    (is (j/duration? (j/duration 1 :days)))
+    (is (not (j/duration? nil)))
     (testing "plus"
       (is (= (j/duration 100000001)
              (j/plus (j/standard-days 1) (j/hours 3) (j/minutes 46) (j/seconds 40) (j/millis 1) (j/nanos 0))

           (j/offset-time (j/instant clock) "UTC")))

    (is (= (j/truncate-to (j/offset-time clock) :millis)
          (j/offset-time (j/to-java-date clock) "UTC")))

    (is (j/offset-time? (j/offset-time (j/zone-id "UTC"))))
    (is (j/offset-time? (j/offset-time +0)))

           (j/offset-date-time (j/instant clock) "UTC")))

    (is (= (j/truncate-to (j/offset-date-time clock) :millis)
           (j/offset-date-time (j/to-java-date clock) "UTC")))

    (is (j/offset-date-time? (j/offset-date-time)))
    (is (not (j/offset-date-time? nil)))

    (j/with-clock (j/system-clock "UTC")
      (is (= (j/offset-date-time 2015)
